Savarez makes a set of strings (silver plated bronze on nylon multifilament) the you can put on any 6 string classical guitar and it will sound an octave lower. They come in scale lengths of 25.5 and 29.5 inches, like the one in the foto above.
So you take, for example any cheap classical style guitar, preferably a hybrid with a cutaway and on-board piezo/preamp and make a mini 6 string base tuned E to E. I havn't tried it, but I mean too. In fact, I want to modify one into a little 4 stringer with proper string spacings and defret the fingerboard. Sweet!
Savarez also makes these in 4 string sets for 32 and 34 inch "classical" bass guitar (read: ABG). I helped them prototype those strings in the 90s on my 32 and 34 inch ABGs.
Careful though, these strings do not have ball ends. They are ment to be tied on, thus the "classical" denomination.
Also, I might add that I don't really like them. They are a bit like the TI acousticore strings. Rather unrully. They break easy too and don't age well. I had them on one of my basses that I wasn't playing for a while. And in the middle of the night I hear "thwak!"...Oh there goes another Savarez string. All by itself. lol
http://www.savarez.fr/anglais/guit-speciale.html#2
